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In 1950, the Italian physicist Enrico Fermi was walking to lunch at the Los Alamos National Laboratory with colleagues from the Manhattan Project. They were discussing a recent spate of UFO reports and, as they sat down to eat, Fermi challenged the company. If the cosmos is full of space-faring aliens,io he said, "Where is everybody?"

In The Reason Why, the veteran science writer John Gribbin answers Fermi's "paradox" by saying that we have seen no sign of aliens because they don't exist. Not, at least, in our Milky Way galaxy - and beyond that, the distances are so vast that it is hardly worth asking. "We are alone, and we had better get used to the idea," he concludes.

The likelihood of intelligent life on other planets has been conditioned since the 1960s by the thinking of the astronomer Frank Drake. His eponymous equation divides the question into its component parts, the probabilities of each of which one might conceivably hope to quantify or at least estimate: how many stars have planets, how many are Earth-like, and so on.

Depending on your taste, the Drake equation is either a logical way of getting purchase on a profound question or an attempt to manufacture knowledge from ignorance. In trying to get a meaningful number by multiplying very big ones, very small ones and very uncertain ones, the Drake equation can seem like guesswork disguised as maths. Gribbin, however, asserts that just about every one of the necessary conditions for intelligent life to emerge has a low, perhaps minuscule, probability. Their combination then makes it highly unlikely that we have any galactic neighbours eagerly trying to make contact.

For instance, only a relatively small part of our galaxy is habitable. The crowded interior is bathed in sterilising radiation from black holes and supernovae. Only stars of a certain age have enough heavy chemical elements to make Earth-like planets and dwellers thereon. Only a few such stars lack partners that pull planetary orbits into extreme shapes, making climate variations unendurably extreme.

The specialness of Earth is particularly apparent in the make-up of our solar system. For example, we are protected from more frequent impacts of asteroids and comets - like the one that seems to have sent the dinosaurs to extinction 65 million years ago - by the immense size of Jupiter. More a failed star than a planet, its gravity sucks up these stray objects. One such, comet Shoemaker-Levy 9, ploughed into the giant planet in 1994, leaving a scar the size of Earth.

Gribbin is especially good on the benign effect of the Moon. The Earth is unusual in having a moon so large in relation to itself. It is now believed to have been created when a proto-Earth stumbled into another planet-like object called Theia, with which it shared an orbit 4.5 billion years ago. The rocky debris clumped to form the Moon, while the traumatised, molten Earth swallowed Theia's iron core to give it an unusually large core today, the source of the strong geomagnetic field that deflects harmful particles streaming from the Sun.

This impact probably left Earth spinning fast (a Venusian day lasts the best part of an Earthly year) and tilted on its axis, from which our seasons ensue. What's more, the Moon's gravity stops this tilt from being righted by the influence of Jupiter. Before the debris coalesced into the lunar globe, its gravity created awesome tides on the more rapidly spinning Earth. These rose and fell several kilometres every two hours or so. Even though the barren Moon was too light to hold an atmosphere of its own, life on Earth would be very different - perhaps impossible - without it.

This "rare Earth" case has been made before, but Gribbin gives the arguments a fresh shine. He assembles them in a legalistic rather than strictly scientific manner. That's to say, he marshals (generally impeccable) science to build a case instead of objectively investigating the possibilities. For example, he predicates a discussion of the "habitable zone" of the solar system - a crucial part of the argument - on the claim that it is reasonable to assume that "life as we know it" requires the presence of liquid water.

That Trekkie-inspired "as we know it" is back-covering, and reminds me of a conference I once attended that was convened to ask if life in the cosmos could exist without water. Speaker after speaker insisted that it could not, since that never happens on Earth, which was of course merely a statement that life adapted to water can't do without it. Now, there are arguments why water might be essential for life anywhere, but they are subtle and not the ones Gribbin casually gives. More to the point, they are still arm-waving and do nothing to dent a counterclaim that it is reasonable to suggest that non-aqueous life is possible.

Such solipsism pervades the book, and is implicit in Fermi's paradox to begin with. It supposes that intelligent life will think as we do now, with a determination to find and populate other inhabited worlds - and, moreover, will have already done so in a way that leaves a mark so prominent that we'll find it within the first 50 years (a comically short span in cosmic terms) of looking.

And for that matter, are we ourselves so determined? Perhaps one might conclude from the parlous state of human space exploration that this is just a phase civilisations grow out of. Worse, since spaceflight seems increasingly likely to be a private enterprise, Gribbin's argument seems to imply that mega-rich philanthropists with a penchant for spaceflight, such as Virgin's Richard Branson and Paul Allen, ex of Microsoft, follow inexorably from the laws of physics.

The same historical determinism colours his belief that space-faring civilisations are a one-shot affair on inhabitable planets. If we foul up after having used all of the surface deposits of fossil fuels, he says, we'll never again be able to claw our way out of a state of barbarism. But this assumes that apocalypse comes only after the oil and coal are exhausted, and moreover that a re-emergent civilisation would stall not at the Stone Age but at the pre-industrial enlightenment. In this definition, a civilisation capable of producing Aristotle, let alone Newton, doesn't qualify as intelligent. The challenge of getting from Newton to Neil Armstrong without plentiful oil is a good premise for a science-fiction novel, but it hardly proves anything else.

Gribbin's account of the chance events that allowed humans to evolve from slime is particularly unpersuasive of any broader conclusions. It sounds increasingly like the kind of enumeration of contingency and coincidence that invites us to marvel at how "unlikely" it is that we ever met our spouses. Once Gribbin starts invoking a highly speculative cometary impact on Venus to explain the Cambrian explosion in which complex life diversified about 540 million years ago, one senses that he is determinedly picking out a precarious path to a foregone conclusion.

None of this is to say that The Reason Why is a bad book. On the contrary, it is as lucid, well researched and enjoyable as Gribbin always is, and supplies a peerless guide to the way stars and planets are formed. And as a polemic, it is entirely justified in being selective with the evidence. Besides, many of Gribbin's astrophysical arguments for the rarity of life are robust. As such, they make a convincing case that the galaxy is not teeming with life that is loftily or mischievously ignoring us.

Yet the book fails to offer any philosophical perspective. Throughout history the specialness of humanity has almost always been asserted as a theological issue, whether to counter Copernicus or Darwin. If Gribbin is right and we just got phenomenally lucky, and the laws of physics are in fact very reluctant to allow matter to become self-aware, this is sufficiently peculiar to warrant more comment. Even atheists might then forgive theologians for taking an interest, just as they do in the "fine-tuning" that seemingly makes physical laws exquisitely geared to support matter and life in the first place. Gribbin can suggest only that, if we're alone in the galaxy, we have an even greater responsibility to our planet.

Philip Ball is a freelance science writer who has written for The Guardian and Nature. His latest book, Unnatural, is published by Bodley Head.